The present invention relates to offshore pipe laying.
Hitherto, offshore pipe-lines have normally been laid on the sea bed using a pipe laying vessel or barge. Pipe sections, typically 40 feet long, are welded to the end of the assembled pipe-line on the barge and the pipe is launched over the stern of the barge as the vessel moves forward.
The part of the pipe between the vessel and the sea bed adopts an S shaped configuration having an upper curve called an overbend and a lower curve called a sagbend. It is important to ensure that there is not excess curvature in the overbend and the sagbend, or else the resulting high stress in the pipe can cause ovalisation, buckling or fracture. Such buckles etc. can be extremely expensive to repair. Typically a supporting structure is employed to support the pipe in the overbend region so as to prevent excess curvature there. Stingers are well known for this purpose and typically employ a buoyant structure for supporting the pipe. However, instead of a buoyant stinger, a fixed and rigid stern ramp is also known for supporting the pipe in the overbend. Such a stern ramp comprises a rigid structure extending from the stern of the pipe laying barge and which remains fixed during pipe laying operations. The ramp is fitted with rollers along its length which are positioned along an arcuate path for supporting pipe launched from the barge as it curves downwardly into the water in the overbend.
Although the overbend of the pipe has to be supported to prevent excess curvature, the sagbend need not be supported. Instead, the curvature in the sagbend can be kept sufficiently low to prevent damage to the pipe by maintaining the pipe under tension, so that the pipe hangs between the barge and the sea bed in a gentle catenary like curve.
Hitherto in normal pipe laying operations, fresh pipe sections are welded to the end of the assembled pipeline on the barge with the barge remaining stationary relative to the sea bed, and similarly the assembled pipe remaining stationary relative to the barge. When a fresh pipe length has been welded on and the joint finished off as required, a length of pipe corresponding to the freshly added on length is launched from the barge by moving the barge forward under the pipe and allowing the pipe to slide off the stern of the barge over the stern ramp or stinger. As mentioned above, if tension is being implied to prevent excess curvature in the sagbend, it is important to ensure that tension is maintained sufficiently high when the pipe is being launched. Hitherto, it has been the practice to allow pipe to move rearwardly off the vessel as the vessel moves forward only when pipe tension exceeds a pre-set upper limit. In this way, tension in the pipe is maintained substantially constant while the pipe is being launched.
For performing pipe laying in this manner, numerous systems and types of machinery have been developed. The barge is typically maintained in station and moved forward when desired by a system of anchors, mooring cables and winches. Tension is maintained in the pipe by machines referred to as "tensioners," which grip the pipe and can operate either to pull pipe in or let pipe out when the measured tension moves outside pre-set limits. Thus, in practice the tensioners can be set to maintain pipe tension substantially constant (within fairly narrow limits), and will pull in the pipe or let out pipe when necessary. Thus, when the laying barge is moved forward, pipe tension would tend to increase. However when the tension reaches the upper limit set for the tensioners, the tensioners will permit pipe to move rearwardly off the lay barge until tension comes back within limits.